ci: remove old .gitea workflows - use .forgejo only

The .gitea/ workflows were outdated and caused conflicts:
- Used runs-on: rust, golang (non-existent runners)
- Complex docker:27-cli setup with TLS (fragile)
- Different secret variable names (FORGEJO_REGISTRY_TOKEN vs REGISTRY_PAT)
- No tests before build

.forgejo/workflows/build.yaml is the clean, working version:
- Simplified docker commands
- Proper runner: docker
- Tests run first
- Cleanup on failure
- No hanging processes

fixtures/refcorpus/small.md

@@ -1,11 +0,0 @@
-# Small File
-
-This is a small test file with minimal content.
-
-## Section A
-
-Content for section A.
-
-## Section B
-
-Content for section B.